[PATCH 2/2 V2] ext4: don't bump nr_to_write if LONG_MAX

[Eric Sandeen <sandeen@redhat.com>]

In some cases we can reach ext4_da_writepages() with
wbc->nr_to_write == LONG_MAX, !range_cyclic, and range_whole=1;
in this case we will try to bump it up by a factor of 8, which
leads to a desired_nr_to_write value of -8.

We still get through the logic without actually changing
wbc->nr_to_write because the other tests which would change
it don't trip due to the negative value, but it seems dangerous
to overflow  desired_nr_to_write in the interim, it's not an 
obvious situation.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
---

(V2 minor commit message edits)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 93497f6..2e72a4a 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3004,9 +3004,11 @@ static int ext4_da_writepages(struct address_space *mapping,
 	 * sbi->max_writeback_mb_bump whichever is smaller.
 	 */
 	max_pages = sbi->s_max_writeback_mb_bump << (20 - PAGE_CACHE_SHIFT);
-	if (!range_cyclic && range_whole)
-		desired_nr_to_write = wbc->nr_to_write * 8;
-	else
+	if (!range_cyclic && range_whole) {
+		desired_nr_to_write = wbc->nr_to_write;
+		if (desired_nr_to_write != LONG_MAX)
+			desired_nr_to_write *= 8;
+	} else
 		desired_nr_to_write = ext4_num_dirty_pages(inode, index,
 							   max_pages);
 	if (desired_nr_to_write > max_pages)


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